The surprising images from the moon Enceladus represent some of the most dramatic evidence yet that water in liquid form may be present beyond the Earth.
Excited by the discovery, some scientists said Enceladus should be added to the short list of places within the solar system most likely to have extraterrestrial life.
Scientists generally agree several ingredients are needed for life to emerge, including water in liquid form and a stable heat source. But so far, the evidence of any large amounts of water in liquid form on celestial objects beyond Earth is circumstantial and indirect, based on scientists' analysis of rocks and other data.
Cassini recently snapped high-resolution images showing geyser-like eruptions of ice particles and water vapor at Enceladus' south pole, scientists said. The pictures do not actually show any water in liquid form, but scientists believe the ice and vapor must be coming from underground reservoirs of water close to the surface.
"We have the smoking gun" that proves the existence of water, said Carolyn Porco, a Cassini imaging scientist from the Space Science Institute in Boulder, Colo.
Torrence Johnson, a Cassini scientist at NASA's Jet Propulsion Laboratory in Pasadena, said this marks the first time that scientists have seen evidence of water in liquid form so close to the surface on another body beyond Earth.
If Enceladus does harbor life, it probably consists of microbes or other primitive organisms capable of living in extreme conditions, scientists say.
The findings were published in Friday's issue of the journal Science.
David Morrison, a senior scientist at NASA's Astrobiology Institute, cautioned against rushing to judgment about whether the tiny moon could support life. "It's certainly interesting, but I don't see how much more you can say beyond that," Morrison said.
Scientists believe Mars and Jupiter's icy moons might have—or once had—conditions hospitable to life. But the evidence of water is indirect. In the case of Mars, scientists have never seen any flowing water. But based on their study of rocks, they believe water once existed there. They say magnetic readings of Jupiter's moon Europa strongly suggest that it has an ocean of water, covered by ice.
Saturn is around 800 million miles from Earth. Enceladus measures 314 miles across and is the shiniest object in the solar system.
Enceladus was long thought to be cold and still, in part because it receives so little sunlight. But scientists now believe it is a geologically active moon that possesses an unusually warm south pole and a significant atmosphere.
The south pole "hot spot" is still frigid by Earth standards. The temperature there is minus-297 degrees—about 20 degrees warmer than the neighboring region.
The water is believed to come from underground reservoirs that are under high pressure. Porco said the venting has probably been going on for at least several thousand years, perhaps indicating a lasting heat source underground.
Cassini found the geysers are mostly made up of water vapor and ice particles with significant amounts of carbon dioxide and trace amounts of methane—all of which probably help to replenish the moon's atmosphere.
The Cassini-Huygens mission is a joint NASA-European Space Agency project. The spacecraft was launched in 1997 and went into orbit around Saturn in 2004, exploring its spectacular rings and many moons. Cassini made three flybys of Enceladus last year and is expected to fly within 220 miles of the moon again in 2008.
NASA
Scientists are intrigued with new discoveries about Enceladus.
